These experiments will compare the morphological sequence of events in a neuron which successfully regenerates its axon, the goldfish retinal ganglion cell, with neurons which fail to regenerate following axotomy, the rat retinal ganglion cell with the hope of identifying the causes of failure. Collateral sprouting by intact habenular axons into the interpeduncular nucleus will be compared in fish and in rat and collateral sprouting and regeneration of habenular axons will be compared in fish. Special attention will be paid to a comparison of synaptogenesis by regenerating and sprouting axons.